The present invention relates to a method of forming a semiconductor diaphragm and, in particular, to a method of forming a silicon diaphragm type pressure sensor.
The semiconductor pressure sensor can be of small size, low cost and high performance as compared with a mechanical pressure sensor using a bourdon tube or a bellows, and a silicon diaphragm type pressure sensor is typical of the semiconductor pressure sensor. The structure of the silicon diaphragm type pressure sensor is disclosed in, for example, "Diaphragm Formation and Pressure Sensitivity in Batch-Fabricated Silicon Pressure Sensors", IEDM Technical Digest, December, 1978, pp. 96-99.
According to this publication, it has been considered to control the thickness of the diaphragm.
However, while it is also very important to make uniform the size and shape of such diaphragms, the importance of obtaining this uniform diaphragm size and shape has not yet been sufficiently recognized.
When forming such diaphragms, generally speaking, an anisotropic etching technique is frequently used. This is because the thickness of the diaphragm can be controlled easily when using this etching technique.
Incidentally, in order to obtain uniform strains generated in four diffused piezoresistors by the pressure applied to a diaphragm, it is desirable that the diaphragm be circular. However, when an Si substrate having a (100) crystallographic plane is selectively etched by the anisotropic etching technique, even if the selective etching is performed using a mask with a circular opening, a completely circular diaphragm is difficult to form because of the etching being a function of the crystallographic direction, so that the diaphragm structure formed has an octagonal or similarly polygonal shape. With this polygonal shape, the distance from the center to the periphery of the diaphragm is different at the edges and vertices thereof, which means that the strain is liable to be irregular over the diaphragm surface, thereby inevitably making variations in the diffused piezoresistors irregular. There is also the problem that the size and shape of the diaphragm varies according to the etching time.